In electronics and electrical engineering, components are usually mounted on boards or on a two-sided metallized insulating material. Two-sided copper-coated ceramic boards are therefore used, for example, in power electronics, where the ceramic board constitutes an insulator with a specified dielectric constant. The components are mounted on the structured metal layer and filled, for example, with casting compound.
Here, very high electrical field strengths occur, particularly at the side edges of the top metal layer, due to the high voltages applied between the contact surfaces. For example, the field strength is up to 50 kV/mm at a voltage of 5000 V. At this field strength, the insulation requirements, which are placed on the power module, can no longer be reliably fulfilled due to the breakdown voltage at the junction with the casting compound, i.e. to the insulator, being exceeded.
FIG. 1 shows a typical construction of an encapsulated circuit element according to the prior art. The AlN substrate shown in FIG. 1, on which the circuit is mounted, is designated by 1. The substrate is a high-quality insulator, usually a ceramic board, which has a dielectric constant of about 10.
The insulator 1 is fixed to a copper baseplate 2 (or AlSiC baseplate), for example, which serves to provide both mechanical stability for the circuit and also thermal connection of the circuit to the outside. The copper baseplate 2 thus secures the components of the circuit on the one hand and, on the other, ensures that heat from the components is conducted away to a heat sink, which is not shown in FIG. 1. At the same time, the AlN substrate is joined to the copper baseplate 2 by means of solder connections 3.
In the form shown, the electronic circuit includes an IGBT 4 and a diode 5, which are rated, for example, for voltages of 1600 V. These components 4 and 5 are connected together, for example, with aluminum thick wire bonding wires 6 and/or by means of metallizing on the insulator. In the case of thick wire bonding, the aluminum wires preferably have a thickness of about 200 to 500 μm.
The whole circuit assembly according to the prior art is encapsulated in a soft casting compound 8, for example made from silicone gel, and subsequently fitted into a housing 9 made of plastic. The plastic housing 9 is preferably fixed directly on the copper baseplate 2 and is filled with a hard casting compound 7. Only the feed conductors with load current contacts 10 are fed out of the plastic housing 9. The load current contacts 10 are also connected to the circuit in the housing 9 by means of solder connections 3.
With a housing of this kind according to the prior art, high field strengths generally occur at the edges, points and structures of voltage-carrying elements, which have a small bending radius. Field strengths, which can exceed the local dielectric strength, occur particularly at the edge of the substrate 14, i.e. at the metallization edge, but also due on the one hand to the geometrical arrangement and on the other, to the material characteristics. The consequence is electrical flashovers, which can cause local damage and thus a loss of insulation of the module.
The profile of the equipotential lines in such a construction according to the prior art is shown in FIG. 2 in an enlarged section of the illustration in FIG. 1. A top and a bottom metal layer 15, 16 are located on the top and bottom surfaces 11, 12 of the AlN ceramic layer 1 resulting in a “sandwich construction”. The bottom metal layer 16 of the construction shown in FIG. 2 can be in thermal contact with the copper baseplate 2 and thus with a heat sink by means of a solder connection.
It can be seen from the density of the equipotential lines 13 in FIG. 2 that a high field strength prevails at the edges 14 in this construction according to the prior art so that uncontrolled discharges occur when a material-dependent breakdown field strength is exceeded, as a result of which the sensitive components of the circuit can be destroyed.
In WO 00/08686 a method is disclosed for shifting the break-down voltage to higher values by reducing the field strength at the edges of the metallization 14. To achieve this, a dielectric layer with a second dielectric constant is provided arranged on the ceramic layer, which borders the top metallization. As a result of this, the maximum field strength at the junction from the substrate (ceramic and metallization) to its environment can be reduced.